Third-Generation (3G) mobile systems, such as for instance Universal Mobile Telecommunications System (UMTS) standardized within the Third-Generation Partnership Project (3GPP), have been based on Wideband Code Division Multiple Access (WCDMA) radio access technology. Today, the 3G systems are being deployed on a broad scale all around the world. After enhancing this technology by introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink (HSUPA), the next major step in evolution of the UMTS standard has brought a combination of Orthogonal Frequency Division Multiplexing (OFDM) for the downlink and Single Carrier Frequency Division Multiplexing Access (SC-FDMA) for the uplink. This system has been named Long-Term Evolution (LTE) since it has been intended to cope with future technology evolutions.
The aim of LTE is to achieve significantly higher data rates compared to HSDPA and HSUPA, to improve the coverage for the high data rates, to significantly reduce latency in the user plane in order to improve the performance of higher layer protocols (for example, TCP), as well as to reduce delay associated with control plane procedures such as, for instance, session setup. Focus has been given to the convergence towards use of Internet Protocol (IP) as a basis for all future services, and, consequently, on the enhancements to the packet-switched (PS) domain. LTE's radio access shall be extremely flexible, using a number of defined channel bandwidths between 1.25 and 20 MHz (contrasted with original UMTS fixed 5 MHz channels). In particular, in the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multi-path interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE (Release 8). In order to suit as many frequency band allocation arrangements as possible, LTE standard supports two different radio frame structures, which are applicable to Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modi of the standard. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP's previous radio access technologies.
The overall architecture of LTE is shown in FIG. 1. A more detailed representation of the architecture of Enhanced UMTS Terrestrial Radio Access Network (E-UTRAN) is provided in FIG. 2. The LTE network is a two-node architecture consisting of access gateways and enhanced network nodes, so-called eNode Bs (eNB). The access gateways handle core network functions, i.e. routing calls and data connections to external networks, and also implement radio access network functions. Thus, the access gateway may be considered as combining the functions performed by Gateway GPRS Support Node (GGSN) and Serving GPRS Support Node (SGSN) in today's 3G networks and radio access network functions, such as for example header compression, ciphering/integrity protection. The eNodeBs provides the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface. The air (radio) interface is an interface between a User Equipment (UE) and an eNodeB. Here, the user equipment may be, for instance, a mobile terminal, a PDA, a portable PC, a PC, or any other apparatus with receiver/transmitter conform to the LTE standard.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
In 3GPP LTE Release 8 as well as 3GPP LTE-A release 10, the following downlink physical channels are defined (cf., for instance, 3GPP TS 36.211 “Physical Channels and Modulations”, Release 8, v. 8.3.0, May 2008, available at http://www.3gpp.org and incorporated herein by reference):                Physical Downlink Shared Channel (PDSCH)        Physical Downlink Control Channel (PDCCH)        Physical Broadcast Channel (PBCH)        Physical Multicast Channel (PMCH)        Physical Control Format Indicator Channel (PCFICH)        Physical HARQ Indicator Channel (PHICH)        
In addition, the following uplink channels are defined:                Physical Uplink Shared Channel (PUSCH)        Physical Uplink Control Channel (PUCCH)        Physical Random Access Channel (PRACH).        
The PDSCH and the PUSCH are utilized for data transport in downlink (DL) and uplink (UL), respectively, and hence designed for high data rates. The PDSCH is designed for the downlink transport, i.e. from eNode B to at least one UE. In general, this physical channel is separated into discrete physical resource blocks and may be shared by a plurality of UEs. The scheduler in eNodeB is responsible for allocation of the corresponding resources, the allocation information is signalized. The PDCCH conveys the UE specific and common control information for downlink and uplink, and the PUCCH conveys the UE specific control information in uplink transmission. Specifically, the PDCCH conveys the Uplink Dedicated Control Information (UL-DCI) and the Downlink Dedicated Control Information (DL-DCI).
In general, a wireless mobile channel in a multi-user system typically suffers from variations of transmission condition. Today's mobile communication systems (for instance GSM, UMTS, cdma200, IS-95, and their evolved versions) use time and/or frequency and/or codes and/or antenna radiation pattern to define physical resources. These resources can be allocated for a transmission for either a single user or divided to a plurality of users. For instance, the transmission time can be subdivided into time periods usually called time slots then may be assigned to different users or for a transmission of data of a single user. The frequency band of such a mobile systems may be subdivided into multiple subbands. The data may be spread using a (quasi) orthogonal spreading code, wherein different data spread by different codes may be transmitted using, for instance, the same frequency and/or time. Another possibility is to use different radiation patterns of the transmitting antenna in order to form beams for transmission of different data on the same frequency, at the same time and/or using the same code.
For instance, in a multi-carrier communication system employing OFDM, such as the system discussed in the “Long Term Evolution” work item of 3GPP, the smallest unit of resources that can be assigned and allocated by the scheduler is called “resource block”. A physical resource block is defined as NsymbolDL consecutive OFDM symbols in the time domain on NscRB consecutive subcarriers in the frequency domain as illustrated in FIG. 3. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbolDL×NscRB resource elements, corresponding to one 0.5 ms slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.7.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference). Moreover, a 1 ms subframes, each consisting of two time slots are defined on the physical layer.
Channel quality information is used in a multi-user communication system to determine the quality of channel resource(s) for one or more users. This information may be used to aid in a multi-user scheduler algorithm of the eNodeB (or other radio-access elements such as a relay node) to assign channel resources to different users, or to adapt link parameters (e.g. modulation scheme, coding rate, or transmit power) so as to exploit the assigned channel resource to its fullest potential. In the ideal case, channel quality information for all resource blocks for all users should be always available to the scheduler so as to take an optimum scheduling decision. However, due to constrained capacity of the feedback channel, it is not feasible to ensure this up-to-date availability of the channel quality information. Therefore, reduction and/or compression techniques are required so as to transmit, for example, channel quality information only for a subset of resource blocks for a given user. In 3GPP LTE, the smallest unit for which channel quality is reported is called a sub-band, which consists of multiple (n) frequency-adjacent resource blocks, which means of n·NBWRB subcarriers.
In 3GPP LTE, there exist three basic elements which may be given as feedback for the channel quality:                Modulation and Coding Scheme Indicator (MCSI), which is also referred to as Channel Quality Indicator (COI) in the 3GPP LTE specifications,        Precoding Matrix Indicator (PMI), and        Rank Indicator (RI).        
The MCSI suggests a modulation and coding scheme that should be employed for downlink transmission to a reporting user equipment, while the PMI points to a precoding matrix/vector that is to be employed for multi-antenna transmission (MIMO) using an assumed transmission matrix rank or a transmission matrix rank that is given by the RI. Details on channel quality reporting and transmission mechanisms can be found in 3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 8)”, version 8.7.0, Section 5.2 and 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)”, version 8.7.0, Section 7.2 (all documents available at http://www.3gpp.org and incorporated herein by reference).
All of these elements are summarized as under the term channel quality feedback herein. Hence, a channel quality feedback can contain any combination of or multiple MCSI, PMI, RI values. Channel quality feedback reports may further contain or consist of metrics such as a channel covariance matrix or elements, channel coefficients, or other suitable metrics as apparent to those skilled in the art.
In 3GPP LTE (Release 8) there are different possibilities defined, how to trigger the user equipments to send channel quality feedback reporting the downlink channel quality. Besides periodic CQI reports (cf. for instance Section 7.2.2 in 3GPP TS 36.213, version 8.7.0), there is also the possibility to use L1/L2 control signaling to a user equipment to request the transmission of the so-called aperiodic CQI report (cf. Section 7.2.1 in 3GPP TS 36.213, version 8.7.0). This L1/L2 control signaling can also be used in the random access procedure (cf. Section 6 in 3GPP TS 36.213, version 8.7.0, incorporated herein by reference). In both these cases, a special CQI request field/bit/flag is included in the control message from the eNodeB/relay node to the user terminal.
The L1/L2 control signaling that conveys information about an uplink assignment is sometimes called UL-DCI. FIG. 4 shows an example of the DCI format 0 for FDD operation as defined in 3GPP TS 36.212, Section 5.3.3.1.1 which serves to convey uplink DCI (please note that the CRC field of DCI format 0 is not shown in FIG. 4 for simplicity. The CQI request flag contains information whether the receiver should transmit CQI within the allocated uplink resources or not. Whenever such a trigger is received, the user subsequently transmits the feedback generally together with uplink data on the assigned Physical Uplink Shared CHannel (PUSCH) resources (the detailed procedure is described in Section 7.2 et seq. in 3GPP TS 36.213, version 8.7.0).
The frequency spectrum for IMT-Advanced was decided at the World Radio-communication Conference 2007 (WRC-07) in November 2008. Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3GPP. At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved which is also referred to as “Release 10”. The study item covers technology components to be considered for the evolution of E-UTRA, for instance, to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
In order to extend the overall system bandwidth, LTE-A (Release 10) uses carrier aggregation, where two or more component carriers are aggregated in order to support wider transmission bandwidths, for example, up to 100 MHz and for spectrum aggregation. It is commonly assumed that a single component carrier does not exceed a bandwidth of 20 MHz. A terminal may then simultaneously receive and/or transmit on one or multiple component carriers depending on its capabilities:                An LTE-Advanced (Release 10) compatible mobile terminal with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers.        An LTE Release 8 compatible mobile terminal can receive and transmit on a single component carrier only, provided that the structure of the component carrier follows the Release 8 specifications.        
It is also envisaged to configure all component carriers LTE Release 8 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are same. Consideration of non-backward-compatible configurations of LTE-A component carriers is not precluded.
As there is only one component carrier defined in LTE Release 8, there is no ambiguity at the user equipment on which portion of the system bandwidth CQI reporting is to be done. The CQI request flag (together with the current transmission mode) is unambiguously indicating to the user equipment how to provide CQI feedback to the eNodeB.
With the introduction of carrier aggregation in LTE-A Release 10 and assuming that the LTE Release 8 CQI reporting procedures should be reused, there are different possibilities how a CQI request can be interpreted by the user equipment.
As shown in FIG. 5, it may be generally assumed that UL-DCI (containing the CQI request) for uplink transmission that is transmitted from an eNodeB or a relay node to a user equipment is placed within a single downlink component carrier (DL CoCa). A simple rule to handle the CQI request at the user equipment would be that whenever a UL-DCI requests a CQI transmission by the user equipment, same applies to the downlink component carrier where the corresponding UL-DCI is transmitted. This means that the user equipment would only send aperiodic CQI feedback in a given uplink transmission for those downlink component carriers that comprised an UL-DCI requesting a CQI report at the same time. In other words, one UL-DCI triggers aperiodic CQI for one single downlink component carrier. Accordingly, in order to request CQI for multiple downlink component carriers, the number of component carriers for which CQI is requested is identical to the number of required transmitted UL-DCI messages. Thus, in order to request CQI for five component carriers it is required to transmit five times more UL-DCI messages than for the case of requesting CQI for just a single component carrier. This solution is therefore not very efficient from the point of view of a downlink control overhead. Moreover, in the particular example of LTE-A Release 10, assuming the reusing of the channel quality feedback reporting mechanism, it is impossible to report channel quality for a component carrier operating without PDCCH, a so called extension carrier, as there is no possibility to transmit any UL-DCI (or indeed, any DCI) on such a component carrier.
An alternative handling of UL-DCI comprising a CQI request is illustrated in FIG. 6. Whenever an UL-DCI requests a CQI transmission by the user equipment, the user equipment applies said request to all downlink component carriers available for downlink transmission to the user equipment. Accordingly, a single UL-DCI message triggers transmitting an aperiodic CQI for all downlink component carriers. Therefore, the downlink control overhead is rather small. However, the resulting uplink transmission always requires a large amount of resources to accommodate the transmission of CQI for all component carriers, even though the network knows that it currently requires CQI only for a single selected component carrier. Therefore, transmitting CQI for all component carriers is not efficient from the point of view of the uplink resource utilization and does not offer any flexibility for the number of requested component carrier CQI.
In a system with aggregated carriers, in which the downlink transmission may occur on multiple component carriers, an efficient scheduling and link adaptation depends on the availability of accurate and up-to-date channel quality feedback. Therefore, in order to make efficient use of the control signaling and channel quality feedback transmission resources, it should be possible to control the number of component carriers for which a channel quality feedback is to be requested and transmitted. Apart of the flexibility, it would be advantageous to provide a system which enables probing of component carriers, which means determining before data transmission whether and/or which component carriers offer sufficient spectral efficiency. Moreover, for the case of existing systems, it would be beneficial to provide the channel quality feedback mechanism in a backward compatible manner, possibly reusing the already existing reporting procedures.